1. Field of the Invention
The present invention relates to high pressure discharge lamps used for general illumination or a light source of a projector. In particular, the present invention relates to high pressure discharge lamps operable with a reduced starting voltage.
2. Description of the Related Art
In recent years, image projection apparatuses such as a liquid crystal projector and a DMD™ (Digital Micromirror Device) projector have been commonly used as systems for realizing large-scale video images. For such an image projection apparatus, in general, an ultrahigh pressure mercury lamp having a high intensity has been commonly used among various high pressure discharge lamps.
Key factors in developing such a projection apparatus are the brightness, size, and weight of the apparatus, and projection apparatuses of a smaller size, a reduced weight and high brightness have been developed.
One of means for attaining a projection apparatus of a small size and a reduced weight is to reduce the starting voltage of a high pressure discharge lamp installed in the apparatus. If the starting voltage can be reduced, a lighting circuit for the lamp can be made small and lightweight with a reduced cost. Furthermore, if the starting voltage can be reduced, the safety against voltage is improved and simpler interconnect wires and connectors can be used, resulting in cost savings. Moreover, influences of the starting voltage on other electronic circuits, such as noises, can be reduced, which brings about the advantage that a possible occurrence of malfunction or failure decreases.
Against the foregoing backdrop, in order to reduce the starting voltage of the high pressure discharge lamp, a method has been generally known in which a trigger wire is wound around the lamp (for example, Japanese Unexamined Patent Publications No. 9-265947 and No. 8-87984). The invention described in each publication is the invention in which a modified method for setting up a trigger wire is provided to improve the demerits caused by setting up the trigger wire. Details on the invention will be described below.
In the invention described in Japanese Unexamined Patent Publication No. 9-265947, a horizontally mounted, DC (direct current)-operated metal halide lamp has the structure in which an end of a trigger wire is connected to a cathode and an intermediate portion of the trigger wire is disposed away from an outer wall of a luminous bulb. According to the description, this structure makes it possible to reduce damages by positive ions drawn toward the trigger wire to quartz forming the luminous bulb.
Japanese Unexamined Patent Publication No. 8-87984 describes a short arc type, DC-operated, mixed metal vapor discharge lamp which includes a bulb consisting of an expansion portion and a pair of branch tube portions. The lamp has the structure in which a trigger wire is connected at one end to a cathode side and stretched to dispose the other end thereof at a position 3 mm or more away from the bulb-rising end of the branch tube portion in an anode unit. According to the description, like Japanese Unexamined Patent Publications No. 9-265947, this structure makes it possible to reduce damages by positive ions drawn toward the trigger wire 7 to quartz forming a luminous bulb.